Quantum information science provides new paradigms of communication, measurement and computation [1]. Some of the most startling future quantum technologies are quantum key distribution, which offers perfectly secure communication; quantum metrology, which allows more precise measurements than could ever be achieved without quantum mechanics and quantum computers, which promise exponentially faster operation for particular tasks. Particularly appealing today are quantum simulators, where one controllable quantum system is used to efficiently investigate the behaviour and properties of another, less accessible one. Quantum simulators hold the promise of tackling problems that are too demanding for classical computers but would require far less resources than a full-scale quantum computer [2].

Recent quantum optical work has highlighted the promise of monolithic integrated optics for quantum information science [3,4,5]. We take advantage of the photonic technology to demonstrate high-fidelity realizations of key quantum photonic circuits [6,7], the building blocks of future quantum simulators for physical, chemical and biological systems [8,9].